1. Field of the Invention
This invention relates to a method for producing an arcuately curved film casing of a thermoplastic synthetic resin material which is suitable for packaging an extrudable material in the form of an arcuately bent cylindrical roll similar to a sausage stuffed in hog's or sheep's intestine.
2. Description of the Prior Art:
Tubular film casings of a thermo-contractive synthetic resin material, for example, of a vinylidene chloride-vinyl chloride coplymer, are presently widley used for packaging foodstuffs such as ham, sausage, boiled fish meat paste, cheese and the like. The hams and sausages packed in these film casings are generally in the form of straight cylindrical rolls, unlike the sausages packed in the hog's or sheep's intestine which have an arcuately curved shape. Arcuately curved rolls of ham or sausage are preferred to straight ones since they give a luxurious appearance when served on a plate or displayed in a show window. For this reason, there have been made various attempts to produce curved rolls of ham and sausages with use of plastic film casings, for example, by heat-setting straight rolls in an arcuate shape in a curved retainer. However, this method encounters difficulties in that the heat-setting operation requires a large amount of additional labor and in that the heat-set casings contain wrinkles to such a degree as to lower the commercial value of the final products.
In order to eliminate these difficulties, attempts have also been made to produce a curved film casing by arcuately deforming a tubular plastic film casing. For example, British Pat. Specification No. 1,163,248 describes a method for producing a curved film casing, where the film is initially formed into a straight tubular form and then curved arcuately by thermal contraction, more specifically, by heating a certain circumferencial portion of the tubular film along the its length. In this connection, another similar method is described in British Pat. Specification No. 1,227,249 where a circumferencial portion of a tubular film is likewise heated along the length of the tube. However, in the latter method, the heating treatment is intended not to cause deformation to the tubular film but to cause variations in thermal contraction coefficient in the circumferencial direction of a transverse section of the film tube such that, when the film casing is heated after being filled, for example, with sausage material, the content of the casing will spontaneously bend arcuately due to the variations in the thermal contraction coefficient of the film.
The above-mentioned prior-art methods, however, have a drawback in that the production process has to include an additional step for the thermal treatment of the casing, which not only requires complicated operation but reduces the thermal contraction coefficient of the film casing since the tubularly preformed thermo-contractive film has to be inevitably subjected to localized heating treatment. The thermal treatment also reduces the mechanical strength of the film, inviting breakage or rupture when filling the casing.
Because of the foregoing, there has been a strong demand for a method which is capable of producing arcuately curved film casings from a synthetic resin material in a more simplified manner.